Tagging of genes by chromosomal integration of PCR amplified cassettes is a widely used and fast method to label proteins in vivo in the yeast

The spindle orientation checkpoint (SPOC) of budding yeast delays mitotic exit when cytoplasmic microtubules (MTs) are defective, causing the spindle to become misaligned. Delay is achieved by maintaining the activity of the Bfa1–Bub2 guanosine triphosphatase–activating protein complex, an inhibitor of mitotic exit. In this study, we show that the spindle pole body (SPB) component Spc72, a transforming acidic coiled coil–like molecule that interacts with the γ-tubulin complex, recruits Kin4 kinase to both SPBs when cytoplasmic MTs are defective. This allows Kin4 to phosphorylate the SPB-associated Bfa1, rendering it resistant to inactivation by Cdc5 polo kinase. Consistently, forced targeting of Kin4 to both SPBs delays mitotic exit even when the anaphase spindle is correctly aligned. Moreover, we present evidence that Spc72 has an additional function in SPOC regulation that is independent of the recruitment of Kin4. Thus, Spc72 provides a missing link between cytoplasmic MT function and components of the SPOC.

The budding yeast spindle aligns along the mother- bud axis through interactions between cytoplasmic microtubules (CMs) and the cell cortex. Kar9, in complex with the EB1-related protein Bim1, mediates contacts of CMs with the cortex of the daughter cell, the bud. Here we established a novel series of events that target Kar9 to the bud cortex. First, Kar9 binds to spindle pole bodies (SPBs) in G(1) of the cell cycle. Secondly, in G(1)/S the yeast Cdk1, Cdc28, associates with SPBs and phosphorylates Kar9. Thirdly, Kar9 and Cdc28 then move from the SPB to the plus end of CMs directed towards the bud. This movement is dependent upon the microtubule motor protein Kip2. Cdc28 activity is required to concentrate Kar9 at the plus end of CMs and hence to establish contacts with the bud cortex. The Cdc28-regulated localization of Kar9 is therefore an integral part of the program that aligns spindles.

The Atf1 transcription factor plays a vital role in the ability of Schizosaccharomyces pombe cells to respond to various stress conditions. It regulates the expression of many genes in a stressdependent manner, and its function is dependent upon the stress-activated MAPK, Sty1/Spc1. Moreover, Atf1 is directly phosphorylated by Sty1. Here we have investigated the role of such phosphorylation. Atf1 protein accumulates following stress, and this accumulation is lost in a strain defective in the Sty1 signaling pathway. In addition, accumulation of a mutant Atf1 protein that can no longer be phosphorylated is lost. Measurement of the half-life of Atf1 demonstrates that changes in Atf1 stability are responsible for this accumulation. Atf1 stability is also regulated by its heterodimeric partner, Pcr1. Similarly, Pcr1 levels are regulated by Atf1. Thus multiple pathways exist that ensure that Atf1 levels are appropriately regulated. Phosphorylation of Atf1 is important for cells to mount a robust response to H 2 O 2 stress, because the Atf1 phospho-mutant displays sensitivity to this stress, and induction of gene expression is lower than that observed in wild-type cells. Surprisingly, however, loss of Atf1 phosphorylation does not lead to the complete loss of stress-activated expression of Atf1 target genes. Accordingly, the Atf1 phospho-mutant does not display the same overall stress sensitivities as the atf1 deletion mutant. Taken together, these data suggest that Sty1 phosphorylation of Atf1 is not required for activation of Atf1 per se but rather for modulating its stability.